Why does current alternate in an AC generator?

Question

I understand how generators work, but I can't for the life of me conceptualize why the current in an AC generator reverses every 180 degrees!!! I understand that, using the right hand rule, it can be seen that the current flows around the coil in a generator in one direction, but I can't see why the current reverses after half a rotation!

I've been looking at the animations on this page to try and figure it out, but I just can't get there.

In addition, I don't understand the concept of how split ring/slip rings work? I know split ring is for direct current, but not really why. For instance, if asked how could I 'explain the difference between a split ring or slip ring'?

Answer 1

Set up a magnet around a coil, such that the Magnet has a field that is constant in magnitude, and has vector form ${\vec B} = B_{0}(\cos (\omega t) {\hat z} + \sin (\omega t) {\hat x})$. Orient the coil so that it lies in the $x-y$ plane and thus has a normal that points in the $z$ direction. If the overlap of the coil's area and the magnetic field is $A$, then the net flux through the coil as a function of time is given by $\Phi=B_{0}A\cos\theta$. Then, Faraday's Law ($V_{ind}=-\frac{d\Phi}{dt}$) tells us that the induced voltage through the loop is given by $V_{ind}=B_{0}A \omega \sin \omega t$, which reverses every half cycle.

By going back to the original inducing magnetic field, you can see that this reverses every half cycle because the direction of the magnetic field also reverses every half cycle.

Answer 2

Slip rings give you unbroken connections to the rotating coil, whereas split rings perform a switching function called "commutation", which in its simplest form is to reverse the outside world's connections to the rotating coil every half revolution.

The voltage induced in a coil rotating in a fixed field will alternate simply because the orientation of the fixed field reverses relative to the rotating coil. Using the right-hand rule, as the "up" end of the coil sweeps past the north pole of the stationary field you get one polarity of voltage, as it sweeps past the south pole, you get the other polarity.

If you connect to the rotating coil with slip rings, which perform no commutation, you will observe this alternating voltage at the output of the machine, and you have an AC source. If you use simple split rings (two contacts each approaching 180 degrees), you will still see time-varying voltage at the output, but the switching action of the rings will cause the negative-going polarity to be switched to a positive-going polarity (or vice-versa, it's a matter of perspective). With a single coil this just barely qualifies as a DC source, but with more coils and more segments in the split ring, better approximations to DC can be had.

Answer 3

I know exactly what you mean. You use the right hand rule to follow the loop of wire around the magnetic field, and the current always flows in the same direction! So then how does the voltage reverse? The trick is in the slip rings; each one is connected to its own wire. So let's say you have a N-facing magnet on the right and an S-facing magnet on the left and a coil spinning through it counter clockwise. If you just focus on the piece of wire that is on it's downward motion (the one close to the S-facing magnet), you'll notice that it is always pushing current towards - in this particular instance - you. For proof, use right hand rule - pointer finger pointing north to south (in this case left), middle finger point to direction of force (in this case down), thus the current is always going towards you. But the fact that the wires going in the downward motion are hooked up to separate slip rings, you soon realize that in one half turn the current is flowing to one side of the voltmeter, and in the other half turn is being sent to the other side. This is the push/pull that is witnessed in AC generation.